Containers for liquids comprising a paper substrate coated with a resin have such characteristics that the containers can keep the qualities of the content for a long period of time in the case of charging liquids in the sterilized containers at a low temperature, and therefore a demand for such containers has been rapidly extended in recent years. Objects of long-term storage include milk, juices, coffee drinks, soups, etc, and they are charged in the containers after subjected to a high-temperature instantaneous sterilization.
The containers for liquids are manufactured by coating both surface sides of a paper substrate with a polyolefin resin and heat-sealing the polyolefin resin laminated on the paper substrate. In the case where liquids having delicate taste such as juice, liquor, etc. are filled into the containers having a polyolefin resin layer as the inmost layer, scent of the content is reduced or taste of the content is deteriorated in many cases. The deterioration of taste of the content in the containers is caused, for example, by that a low-molecular compound contained in the inmost layer of polyolefin resin or a decomposition product or volatile component produced during the lamination of the resin is transferred to the content in accordance with a rise of the liquid temperature, or the taste or scent of the content is reduced owing to the adsorptivity of the resin itself. Further, the polyolefin resin has a permeability, so that external odor transfers into the content to produce foreign taste or foreign odor, or the scent of the content escapes outside.
Various containers improved in the above-mentioned defects of the polyolefin resins have been now developed. For example, a container having a low-crystalline or non-crystalline polyester resin layer as the inmost layer (Japanese Patent Laid-open Publications No. 55(1980)-166247, No. 56(1981)-24165, No. 62(1987)-290534, etc.) or a container having a terpolymer polyester resin layer as the inmost layer (Japanese Patent Laid-open Publication No. 59(1984)-59435) are proposed as a container improved in the scent adsorption.
Paper containers for liquids now employed are roughly classified into two types, that is, so-called brick type containers and gable type containers. Processes for hydrogen peroxide sterilization treatments of those containers are different from each other. In the case of the brick type containers, a process of continuously immersing the laminate material in a hot hydrogen peroxide liquid and then drying it is mainly carried out.
In the case of gable type containers, on the other hand, there is carried out a process of cutting the laminate material into a carton-extended shape, heat-sealing the transverse cross section to prepare a semi-finished carton, further heat-sealing its bottom portion and then spraying it with hydrogen peroxide, or a process of placing a carton in vapored hydrogen peroxide to produce dropwise condensation of the hydrogen peroxide (Japanese Patent Laid-open Publication No. 63(1988)-11163) and then drying the carton.
However, the present inventors have found that containers having an inside layer made of a polyester resin well known in the art as a non-crystalline polyester resin having high scent retention properties (trade name: PETG6763, available from Eastman Kodak) are markedly reduced in the heat-sealing properties when the containers are subjected to the above-mentioned hydrogen peroxide sterilization treatment.
As described above, the packaging laminate materials comprising a polyolefin resin are deteriorated in the scent retention properties, and the packaging laminate materials comprising a polyester resin are reduced in the heat-sealing properties by the hydrogen peroxide sterilization treatment, though they are good in the scent retention properties as compared with those laminated with a polyolefin resin.
The present invention intends to solve the above-mentioned problems in the prior art, and the object of the invention is to provide a packaging laminate material which is excellent in the heat-sealing properties and the scent retention properties and is hardly reduced in the heat-sealing properties even after subjected to a hot water treatment or a hot hydrogen peroxide treatment.